Laser scanning imaging apparatus and method of ranging

ABSTRACT

A laser beam is projected through the optical system of an endoscope and scanned in raster fashion by means of a scanning head at a proximal end of the endoscope tube. The beam is projected from a distal end of the endoscope tube so as to be scanned over an object. Light reflected from the object is detected and used to form a television image. The range of the object is measurable in a ranging mode of the apparatus in which the depth of focus of the projected laser beam is reduced using a zoom lens at the distal end of the endoscope and the focus distance of the projected beam varied until an in-focus position is detected by analysis of the reflected position is detected by analysis of the reflected light. Detection of the in-focus position relies on characteristics of laser speckle, a selected portion of the object being scanned in ranging mode to detect maximum modulation in the speckle pattern which occurs when the focus distance corresponds to the range of the object. Range and object size information may then be included in a television image of the object.

BACKGROUND OF THE INVENTION

The present invention relates to optical apparatus and in particular butnot exclusively to electronic apparatus adapted for insertion intoconfined spaces to obtain a television image of an inaccessible object.

Such apparatus are well-known and are used as endoscopes to examineinternal surfaces of, for example, human bodies and as borescopes toexamine mechanical components such as engines, aero engines, andaircraft.

Whilst generally satisfactory there are a number of difficulties inutilizing such known apparatus. In particular, it can be difficult toobtain an image of sufficient contrast and resolution and furthermore,particularly when being used in an unfamiliar environment, it isdifficult to obtain from the image produced an indication of the size ofthe objects being viewed.

It is known from EP0084435 to improve the contrast and resolution of anoptical apparatus by providing means for focusing the optical system ofthe apparatus in which an intense spot of light is projected onto anobject and the focus is adjusted until a clear image of the spot isformed. A disadvantage of such apparatus is that in some applications ahigh intensity source cannot be used and also the technique is notreadily adaptable for use in an electronic imaging apparatus.

It is also known to provide a laser scanning camera for remoteinspection purposes in which a laser beam is projected onto an objectfield and scanned in raster fashion, the reflected beam being detectedand used to form a television image. A disadvantage of such cameras isthat their bulk inhibits use in confined spaces.

It is also known to use an electronic imaging apparatus to provide atelevision image in which a solid state detector such as a chargecoupled device is mounted at the tip of an apparatus tube. Adisadvantage of such apparatus is that they cannot be used in certainhazardous environments where the tip is to be exposed to high levels ofradiation such as in the inspection of nuclear reactors.

Throughout the specification we will refer to "light", "optical" andlike expressions. It will be understood, however, that the presentinvention is not restricted to electromagnetic radiation of visiblewavelengths, but may apply to other wavelengths such as infra-red andultraviolet. The term "lens" used in the specification should also beunderstood to encompass groups of lens elements where appropriate.

SUMMARY OF THE INVENTION

According to the present invention there is disclosed a method of rangemeasurement comprising the steps of passing a laser beam through anoptical system so as to be projected into an object field containing anobject to be ranged, detecting light reflected from the object, varyingthe focus distance between an output of the optical system and theposition at which the beam is focused by operation of a focus distancevarying means, measuring a parameter of the detected light which ischaracteristic of laser speckle, determining a setting of the focusdistance varying means at which the value of the speckle parameter isconsistent with the beam being focused onto the object, and determiningfrom calibration of the focus distance varying means the correspondingvalue of focus distance as a measurement of range, wherein the opticalsystem further comprises an optical relay, the method including the stepof passing the laser beam through the optical relay, which optical relaydefines an optical axis extending longitudinally through an elongatetube, the output of the optical system being located at a distal end ofthe tube which is insertable into confined spaces for range measurementof inaccessible objects.

According to a further aspect of the present invention there isdisclosed imaging apparatus comprising an optical system defining anoptical axis, a scanning head connected to an input of the opticalsystem, a laser light source connected to an input of the scanning head,the scanning head being operable to project a laser beam into the inputof the optical system such that the angle between the beam and the axisis scanned in raster fashion, the optical system having an outputcomprising a light transmitting window from which the laser beam isprojected towards an object field, a light receiving window locatedadjacent the transmitting window for collecting light reflected from theobject field, detection means producing an electrical output signalresponsive to light collected by the light receiving window, andelectronic apparatus operable to produce a television signal from thedetector means output signal whereby a television image of an object inthe object field may be obtained, wherein the optical system comprisesan elongate tube, the transmitting window and the receiving window beingmounted in a distal end of the tube, the optical system including anoptical relay through which the optical axis extends longitudinallywithin the tube, and wherein the tube is of small cross-section relativeto the scanning head so as to be insertable into confined spaces forimaging inaccessible objects.

According to a further aspect of the present invention there isdisclosed an optical assembly comprising an objective lens and a fieldlens which is movable relative to the objective lens between a firstposition adjacent the objective lens and a second position spaced fromthe objective lens, the field lens and objective lens in combinationdefining a focal plane which is at the same location relative to theobjective lens for both first and second positions of the field lens andwherein the field lens passes through the focal plane in moving betweenthe first and second positions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a folded side view of apparatus including a forward viewingendoscope;

FIG. 2 is an end view of part of the apparatus of FIG. 1;

FIG. 3 is a schematic view of an alternative apparatus including a sideviewing endoscope;

FIG. 4 is a detail of the tip of the endoscope of FIG. 3;

FIG. 5 is a perspective view of the internal components of a scanninghead of the apparatus of FIG. 3;

FIG. 6 is a schematic diagram of a mirror system of the scanning head ofFIG. 5;

FIG. 7 is a diagram showing a zoom lens of the endoscope of FIG. 3;

FIG. 8 is a diagram of the zoom lens of FIG. 7 in its rangingconfiguration;

FIG. 9 is a schematic diagram of a control apparatus of the apparatus ofFIG. 3;

FIG. 10 is a perspective view of the proximal end of a forward viewingendoscope in a modified apparatus;

FIG. 11 is a schematic plan of the proximal end of the apparatus of FIG.10;

FIG. 12 is a schematic view of a conventional zoom lens incorporated ina modification to the apparatus of FIG. 3;

FIG. 13 is a schematic view of the zoom lens of FIG. 12 in its rangingconfiguration;

FIG. 14 is a schematic view of a further zoom lens with field curvaturecorrection;

FIG. 15 is a schematic view of the zoom lens of FIG. 14 in its rangingconfiguration;

FIG. 16 is a schematic plan of a proximal end of a modification to theapparatus of FIG. 3 in which the zoom lens is replaced by a fixed lenssystem and optical switching means;

FIG. 17 is a schematic view of an otherwise conventional endoscopemodified to include a zoom lens;

FIG. 18 is a schematic diagram of a laser scanning camera; and

FIG. 19 is a perspective view of the scanning unit of the camera of FIG.18.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An apparatus including optical apparatus for use as an endoscope orborescope illustrating a preferred embodiment of several aspects of theinvention will now be described by way of example only and withreference to the accompanying drawings. Reference to the term"endoscope" should be understood to encompass the same apparatus used innon-medical uses as a borescope.

Referring to FIGS. 1 and 2 there is shown in diagrammatic sketch form anapparatus comprising an endoscope 10.

In FIG. 1 the length of endoscope 10 is chosen for the particularpurpose required. The endoscope 10 comprises a rigid hollow tube 11 of10 mm diameter containing an optical relay comprising a series of relaylenses 12 spaced along the tube so that the image plane of one coincideswith the object plane of the next lens in the series.

Surrounding the optical relay is a longitudinally extending fibre opticbundle 13 of annular cross-section.

The endoscope 10 has a distal end 36 presented to an object 28 to beviewed and a proximal end 35 at which there is provided a laser scanninghead 39 including a laser 14. A laser beam 15 passes from the laser 14to a first scanning mirror 16 which is pivoted for scanning through asmall angle about an axis X passing through the point at which the laserbeam 15 strikes the mirror 16 such that the beam 15 is at 45° to themean position of the mirror. The beam reflected from the scanning mirror16 passes to a second scanning mirror 18 via a lens 17, which mirror 18is pivoted for scanning through a small angle about an axis Y at rightangles to the axis X such that the beam is at 45° to the mean positionof the mirror.

The beam 15 reflected from the second scanning mirror 18 passes to acollecting lens group 19 comprising two spaced apart lenses directingthe beam into the proximal end 35 of the endoscope 10 where it enters aninput lens 21 of the optical relay 12.

The lens 17 is arranged so as to bring the laser beam 15 into focus inan image plane 22 located between the two lenses of the lens group 19 sothat any dust on the lens surfaces does not interfere with the image.This image plane 22 is arranged to be the object plane of the first lens21 of the optical relay lenses 12.

An output lens assembly 23 is provided at the distal end 36 of theendoscope 10 and has a projected image plane 34 which is illustrated inFIG. 1 as being coincident with the object 28.

A light collecting input surface 26 of the fibre optic bundle 1surrounds the output lens assembly 23 and faces the object 28 so thatlight reflected from the object is collected at the surface and passesinto the fibre optic bundle. The collected light is conveyed along thefibre optic bundle 13 to a photomultiplier 27 connected by a fibre opticlink 37 to the proximal end 35 of the endoscope 10.

An interference filter 93 transmitting light only at the laser frequencyis placed in front of the photomultiplier 27 to remove effects ofambient light.

The output signal from the photomultiplier 27 is passed to an electronicapparatus 29 which is synchronised with the scanning mirrors 16,18 toform a television image viewed on a television monitor screen 33.

In addition to the described components there is provided in thescanning head 39 a pair of lenses 31,32 which may selectively be movedinto and out of the path of laser beam 15 between the first scanningmirror 16 and lens 17, this movement and their exact positions along theoptical axis being controlled by a motor system 30. In FIG. 1 the lenses31,32 are shown in broken lines at their positions when moved into thepath of the laser beam 15 and are shown in unbroken lines at theirnormally retracted positions in which they are clear of the path of thelaser beam 15.

The lenses 31,32 consist of a diverging lens 31 followed by a converginglens 32 which when introduced into the optical path have the effect ofbroadening the beam such that on emerging from the distal end 36 aprojected laser beam 38 fills the aperture defined by the output lens23. When retracted from the optical path the effective aperture of theapparatus 1 is reduced or in other words the beam 38 remains relativelynarrow and is projected from the output lens 23 with a reducedconvergence angle. Axial movement of the lenses 31,32 provides movementof the focal plane 34 at which the projected beam 38 is focused.

In use of the apparatus 1 the laser beam 15 is scanned horizontally andvertically across the image plane 22 by means of the first and secondscanning mirrors 16,18 respectively in a raster form. Thus, a spot imageof the beam 15 formed in the image plane 22 moves horizontally line byline and slowly vertically downwards as in a television picture. Thelimits of the range of movement of the beam 15 are generally illustratedin FIG. 1, but it will be understood that at any one time the beam 15 isdeflected to a single point in the image plane 22. The terms verticaland horizontal used with reference to the raster scan are relative anddo not necessarily comply with those particular directions in space.

The raster scan of the beam 15 is transmitted from the image plane 22,which forms the object plane of the first lens 21 of the optical relay12, and hence is passed into the endoscope 10 and is transmitted throughthe endoscope to the object plane 24 of the output lens assembly 23. Theprojected laser beam 38 is then projected from the endoscope through theoutput lens assembly 23 so as to scan across the object 28 to be viewedin raster fashion (the beam at this point in FIG. 1 being shown in solidlines).

Thus, as the laser beam 38 scans across the object 28 reflected andscattered light is collected by the input surface 26 of the fibre opticbundle 13. The collected light passes down the fibre optic bundle 13 andis detected by the photomultiplier 27.

The photomultiplier 27 at any one time produces a single signaldependent upon the total amount of light entering the input surface 26.

The output signal from the photomultiplier 27 is passed to theelectronic apparatus 29 and a corresponding image point is generated onthe screen 33 which scans at the same rate and in the same way as thebeam 38 is scanned across the object 28. Thus a composite image of thatobject 28 will be produced on the television screen 33 in each completedframe of the scan.

There are a number of advantages of producing a composite image in thismanner. By the use of a laser beam, the light flux illuminating eachelement of the object under examination can be high. All of the light isconcentrated on the spot, rather than generally illuminating the objectunder examination.

Because of the small convergence angle of the projected beam duringimaging it is possible to have great depth of focus which reduces theaberration effects.

Similarly, the use of a laser enables the detected signal to have aninherently high signal to noise ratio. A monochromatic laser source canbe matched to the coatings on the lenses so as to reduce to a very smalllevel the reflection of the beam from the surface of the lenses and,indeed, the lenses themselves can be matched to the particularwavelength of the laser beam.

Furthermore, a clear high contrast high resolution image can be providedwith relatively low input of total energy to the object underexamination which is a considerable advantage in some applications, forexample medical applications. Other applications include imaging ofhazardous environments where high energy illumination must be avoidedbecause of the flammability of vapours for example.

The apparatus 1 may also be used to measure and display information asto the range and size of objects imaged on the screen 33. This isparticularly useful when the user is unfamiliar with the object beingviewed so that it is difficult to gauge an impression of size from thetelevision image in which a small object at a short range could appearto have the same size as a large object at a greater range.

The apparatus 1 is used to measure range of a viewed object 28 in itsrange finding mode in which lenses 31 and 32 are introduced into thepath of laser beam 15. The range finding procedure requires that thefocus distance D between the focal plane 34 and the distal end 36 bevaried until an analysis of the output of photomultiplier 27 indicatesthat the object 28 is in focus i.e. that the projected laser beam 38 isfocused on to a surface of the object 28. Movement of the focal plane 34is accomplished by axial movement of the lenses 31 and 32. Analysis ofthe output of the photomultiplier 27 to determine the in-focus conditionrelies upon the phenomenon of laser speckle which is the randomintensity distribution exhibited in light observed in non-specularreflection of a laser beam and results from constructive and destructiveinterference of coherent waveforms reflected from surfaces which arerough on the scale of the wavelength used. It is observed that thespeckle pattern which appears on the reflecting surface changes as thebeam is brought in and out of focus. Both the spacial frequency andamplitude of intensity modulation are seen to vary, the amplitude ofmodulation being at a maximum when the beam is in focus.

Speckle is detected in the output of the photomultiplier 27 as a strongAC modulation during a horizontal scan which is readily distinguishablefrom the less noisy DC signal resulting from a scan of the same objectwhen the laser beam is out of focus. The beam is taken to be focused onthe object when the amplitude of modulation is at a maximum. Thefrequency of modulation depends upon the rate at which the projectedbeam traverses the object surface, the distance across the reflectingsurface traversed in one cycle of modulation being of the same order ofmagnitude as the cross-section of the projected beam 38 measured at theoutput lens assembly 23.

Effectiveness of the range finding mode is enhanced by the increasedeffective aperture resulting from insertion of the lenses 31 and 32since this results in a marked decrease in the depth of focus of theprojected beam 38.

The projected beam 38 in ranging mode of the apparatus 1 is shown inbroken lines in FIG. 1 in which it is focused on to the object 28.

The distance D corresponding to the position of lenses 31 and 32 is thencomputed by the electronic apparatus 29 and displayed on screen 33 as ameasurement of range R between the object 28 and the distal end 36. Itis then possible to measure the width of a particular object at thatdistance by measuring the width of the image of the object on the screen33 and multiplying this width by a conversion factor based oncalibration measurements.

During ranging the pivotal movement of the scanning mirror 16 isstopped. In addition, because it is necessary to range over only a smallpart of the object, (since otherwise a variable range will be provided)the scanning movement of the mirror 18 is restricted.

The beam used during the ranging operation is illustrated by brokenlines in FIG. 1 and as can be seen it is considerably wider at the lensassembly 23 than the beam represented in unbroken lines used duringnormal imaging.

It has been found optimum to arrange that the overall cross-section ofthe input surface 26 of the optical fibres 12 is the same as thecross-sectional area of the projected beam 38 at the output lensassembly 23 during the ranging operation and under these circumstancesit has been found that as the focal plane 34 is moved back and forth (byrelative movement of lenses 31 and 32) the modulation due to speckle inthe signal detected by the photomultiplier 27 is at a maximum when theplane 34 coincides with the surface of the object 28.

As a result, this provides an automatic method of ranging in whichmovement of the lenses 31,32 to move the plane 34 is controlled by themotor system 30, the operation of which is under control of theelectronic apparatus 29. Thus, in use, the electronic apparatus 29causes the lenses 31,32 to move the plane 34 until the modulation of thesignal detected by the photodetector 27 is a maximum. At that point theelectronic apparatus 29 is able to determine the range R, that is thedistance between the distal end 36 of the endoscope 10 and the object28, by reference to stored data of focus distance D as a function of theposition of lenses 31 and 32 from previous calibration experiments. Whenswitched back to viewing mode the screen 33 may then carry, in additionto an image of the object 28, a scale from which the size of the object28 may be directly measured.

To prevent mismeasurement, the arrangement may be such that when theendoscope 10 is moved the scale on the screen 33 disappears, since atthat point a new ranging will be required.

The invention is not restricted to the details of the foregoing example.In particular in relation to the latter inventive concept, it wouldclearly be useful to regularly range and view. Thus, it may be providedthat the lenses 31,32 are regularly moved into the beam path.Alternatively the beam path itself may be split so as to pass, in oneparallel beam path through the lenses 31,32 and in the other not throughthose lenses, optical switching means being provided to switch back andforth between these two paths whereby the ranging can be regularlycarried out. Hence the scale can be shown on the screen 33 continuouslyalthough it will vary continuously as the endoscope 10 is moved withrespect to the object 28.

The fibre optic bundle 13 is shown in FIG. 1 as being annular but in analternative arrangement the bundle may be of generally circularcross-section.

The ranging and hence scaling of the image has been described withrespect to an endoscope 10. Clearly, however, this aspect of theinvention may be applicable to other optical systems such asmicroscopes, telescopes and other scanning systems.

A further embodiment of the invention is shown in FIG. 3 in whichcorresponding reference numerals are used to those of FIGS. 1 and 2where appropriate for corresponding elements.

The alternative apparatus 50 of FIG. 3 comprises a rigid endoscope tube11 having a distal end 36 which is insertable into inaccessible areasfor imaging purposes and is shown being directed towards an object 28.

The endoscope tube 11 has a proximal end 35 connected to a laserscanning head 39. A flexible single mode fibre optic link 51 connects alaser 14 to the laser scanning head 39, the link 51 comprising a singleoptical fibre which retains the coherence characteristics of the lightcarried.

As shown in FIG. 4 the endoscope tube contains an optical relay 12 forconveying a raster scanned laser beam from the laser scanning head 39 toa zoom lens assembly 52. A right angle prism 81 is arranged to deflectlight emerging from the zoom lens assembly 52 so as to exit through awindow 56 provided in a side wall 55 of the tube 11 as a projected laserbeam 38.

The prism 81 is provided with a prism rotating mechanism 87 capable oftilting the prism about a vertical axis to produce horizontal deflectionof the field scanned by the projected beam 38. Tilting of the prism 81is actuated by a control wire (not shown) connected to an actuator (notshown) in the scanning head 39. Vertical deflection of the field scannedby the projected beam 38 is provided by rotation of the tube 11 aboutits longitudinal axis by means of a tube rotating mechanism (not shown).As described later, the prism rotating mechanism 87 and tube rotatingmechanism provide means for varying the direction of the optical axis atthe window 56 when the projected beam 38 is to be directed onto aselected area of the object which would otherwise be located outside ofthe reduced field angle available in ranging mode.

A fibre optic bundle 53 extends longitudinally through the tube 11 andterminates in a circular light collecting surface 54 forming part of theside wall 55.

The light collecting surface 54 is positioned side-by-side relative tothe window 56 so that the apparatus 50 constitutes a side viewingendoscope.

A control wire 57 extends through the tube 11 and connects the zoom lensassembly 52 with a linear drive mechanism 91 shown in FIG. 5 in thelaser scanning head 39.

A fibre optic link 37 connects the scanning head 39 to a photomultiplier27, the connection being such that substantially all of the lightcollected at the surface 54 is conducted to the photomultiplier. Aninterference filter 93 transmitting light only at the laser frequency ispositioned in front of the photomultiplier 27 to remove the effects ofambient light. An output signal from the photomultiplier 27 is input toan electronic apparatus 29 which in turn produces a signal driving atelevision monitor 58 having a screen 33.

The construction of the laser scanning head 39 is illustrated in FIG. 5and comprises a housing 59 to which is rigidly connected the endoscopetube 11. The fibre optic link 37 is shown projecting from the proximalend 35 of the tube 11.

The single mode fibre optic link 51 carrying the laser beam from laser14 enters the housing 59 and has a terminal 60 which incorporates aconverging lens (not shown). A laser beam 15 emerges from the terminal60 and is represented in the drawing by a single line extending alongthe optical axis of the scanning head 39 whereas in practice the beamwill have finite width and convergence.

The beam 15 is directed on to a plane mirror 61 which is provided tofold the light path and direct the beam into a mirror system 62represented schematically as a cube in the drawing. The mirror system 62is shown in FIG. 6 and consists of two orthogonal mirrors 63 and 64which are fixed relative to each other and mounted at the end of aradius arm 65 providing rotation about a pivot 66. A motor drive andtransducer responsive to angular position (not shown) are provided by arange scanning unit 114 (see FIG. 5) for controlling the angularposition of the mirrors 63 and 64. The effect of shifting position ofthe mirrors by pivotal motion is to increase or decrease the path lengthtravelled by the converging laser beam 15. Consequently the mirrorsystem 62 provides a means of shifting along the optical axis an imageposition 67 at which laser beam 15 is focused by the lens of theterminal 60. This in turn enables the focus distance D at which theprojected beam 38 is focused relative to the window 56 to be dynamicallyvaried.

The laser beam 15 on leaving the mirror system 62 passes through aconverging lens 68 to a scanning mirror 69 arranged to produce scanningmotion in the horizontal direction in the projected beam 38 emergingfrom the end window 56. The scanning mirror 69 is driven by agalvanometer type scanning unit 70 capable of positioning the mirror inany required position in a randomly addressable mode. This facility isrequired during ranging operation as described later.

The beam is then directed by a lens pair 71 to a further scanning mirror16 driven by a further scanning unit 72 of a resonant galvanometer typeproducing horizontal line scan in the projected beam 38 a horizontalscanning frequency of 8 kHz. A cable 76 (represented in FIG. 3) connectsthe scanning head 39 to the electronic apparatus 29 and carries controlsignals.

From the scanning mirror 16 the beam 15 is relayed by a lens pair 73 toa final scanning mirror 18 driven by a final scanning unit 75 of agalvanometer type mechanism providing accurate control of the mirrorposition in a randomly addressable manner. The final scanning mirror 18is arranged to produce vertical scanning of the projected beam 38.

A further lens pair 19 directs the scanned beam 15 into the opticalrelay 12 so that the beam is directed through the endoscope tube 11 intothe zoom lens assembly 52.

The zoom lens assembly 52 is illustrated in detail in FIGS. 7 and 8. Thelens assembly 52 is referred to as a "zoom" lens assembly to convey themeaning that the lens assembly has a variable focal length. In thepresent context it is intended that the focus need not be continuouslyvariable. The present requirement is that the lens assembly shouldprovide two possible configurations giving different focal lengths andhence two possible values of angular magnification, for each of whichconfigurations the optics are well corrected in terms of aberrations.The zoom lens assembly 52 in its normal configuration when the projectedbeam 38 is being scanned for imaging purposes provides a field angle of50° over which the beam is scanned both horizontally and vertically.This configuration will be referred to as the imaging configuration ofthe zoom lens assembly 52.

The zoom lens assembly 52 has a second configuration which will bereferred to as the ranging configuration in which the field angle isreduced to 10°. In the imaging configuration the convergence angle ofthe projected beam 38 is relatively small as shown in solid lines inFIG. 4 whereas in the ranging configuration the projected beam 38 fillsthe aperture defined by the end window 56 and has a greater convergenceangle and consequently provides a reduced depth of field during ranging.

The zoom lens 52 is shown in its imaging configuration in FIG. 7 and inits ranging configuration in FIG. 8. The zoom lens 52 comprises a fixedlens 77 of positive power and comprising compound lens elements 78 and79. An axially movable lens 80 is located between the fixed lens 77 andthe optical relay 12 so that the laser beam 15 passes from the opticalrelay, through the movable lens and then through the fixed lens 77. Aright angle prism 81 deflects the beam 15 emerging from the fixed lens77 through 90°, the light rays drawn to the right of line A--A shouldtherefore be imagined as travelling in a plane orthogonal to the page.

The movable lens 80 and the fixed lens 77 constitute a field lens andobjective lens respectively of the zoom lens.

The movable lens 80 is formed of BK7 glass and comprises a convex singlethick element of 9.56 mm thickness having the same magnitude ofcurvature (0.17357 mm-1) on both its front and rear optical faces 82 and83 respectively.

A fixed lens 77 comprises (from left to right in FIGS. 7 and 8) anelement of SK10 glass of 1.5 mm thickness and curvature 0.11717 and-0.10386; an air gap of 0.10: an element of SK10 glass of thickness 1.50and curvature 0.16440 and -0.00530; a further air gap of 0.10; anelement of SK10 glass with thickness 4.0 and curvature -0.03633 and0.20974; and an air gap of 1.0 following by the prism 81 (all dimensionsof separations being in mm and curvature in mm-1)

The beam 15 is represented by groups of pencil rays 84 and 85 whichcorrespond respectively to a beam projected along the optical axis ofthe apparatus 50 and a beam which is deflected by the scanning head 39so as to form a projected beam 38 which is vertically deflected to themaximum limit available.

The optical relay 12 projects a real image at an image plane 86 which inthe imaging configuration of the zoom lens 52 coincides with the frontface 82 of the movable lens 80 which is also arranged to coincide withthe focal plane of the zoom lens 52 i.e. the combination of the movablelens 80 and fixed lens 77.

In this configuration a wide field angle is provided by the zoom lens 52so that the vertical and horizontal scan in each case covers 50°.

In FIG. 8 the zoom lens 52 is shown in its ranging configuration inwhich the moving lens 80 has been moved away from the fixed lens 77 soas to pass through the image plane 86 projected by the optical relay 12to a new position in which the real image plane 86a coincides with therear face 83 of the movable lens 80.

The image plane 86 is translated by the presence of the movable lens 80in a direction towards the optical relay 12 but in its new position 86aas shown in FIG. 8 is coincident with the focal plane of the fixed lens77.

In FIG. 8 three pencil rays 84 represent the laser beam projected by thescanning head 39 along the optical axis of the apparatus 50 and pencilrays 85 represent the beam when vertically deflected to the maximumavailable limit.

In both FIGS. 7 and 8 the pencil rays 84 and 85 are drawn in the casewhere the projected beam 38 is collimated i.e. focused at infinity sothat for each position of the movable lens 80 the image plane 86corresponds to the focal plane of the combined lens groups 77, 80. InFIG. 8 the focal plane of the fixed lens and movable lens in combinationis omitted for clarity but is a virtual image plane at the same positionrelative to the fixed lens 77 as the real image focal plane 86 in FIG.7.

In FIG. 8 the zoom lens in its ranging configuration provides a fieldangle of 10° for horizontal and vertical scanning of the projected beam38.

A comparision of FIGS. 7 and 8 shows that in the imaging configurationthe projected beam 38 emerges as a tight bundle of rays 84 or 85 whereasin FIG. 7 in the ranging configuration the bundle of rays 84 or 85spreads to fully occupy the available aperture defined by the prism 81.In other words the zoom lens 52 provides an increased exit pupildiameter in the ranging configuration and this results in a decrease inthe depth of focus available in the projected beam 38.

The movable lens 80 is constructed such that the radii of the front andrear faces 82 and 83 are given by the expression ##EQU1## where r is theradius, t is the lens thickness and n is the refractive index of thelens material.

In this arrangement very little aberration is produced, the onlysignificant aberration being pure field curvature when monochromaticlight is used.

The ratio of field angles and pupil diameters at the two configurationsis approximately n⁴.

An advantage of this arrangement is that the exit pupil of the apparatus50 including the zoom lens 52 is close to the prism 81 so that the prismcan be of relatively small size. By contrast a conventional zoom lenswould result in the exit pupil being spaced at a greater distance fromthe prism since conventional zoom lenses include a divergent lens in thefinal stage. An advantage of being able to use a relatively small prismin the apparatus 50 is that only limited space is available within thetube to accommodate the fibre optic bundle 53 and the rotating mechanismof the prism.

In use of the apparatus 50 to form an image of an object 28 in aninaccessible location the endoscope tube 11 is positioned with thewindow 56 facing the object 28 and laser beam 15 projected from thewindow as projected beam 38 to illuminate the object. Reflected lightfrom the object 28 is received at the surface 54 and conducted via thefibre optic bundle 53 to the photomultiplier 27.

The scanning head 39 is set to operate in imaging mode in which scanningmirror 69 remains stationary, scanning mirror 16 is oscillated at itsresonant frequency by the scanning unit 72 so as to provide horizontalscanning deflection of the projected beam 38 and the final scanningmirror 74 is scanned so as to produce a relatively slow verticalscanning motion. A raster scan is thereby produced in the projected beam38 so that the surface of object 28 is systematically scanned by thelaser beam. The output signal of photomultiplier 27 is processed by theelectronic apparatus 29 to produce a television image. An image isdisplayed on screen 33 of object 28.

To measure the range of object 28 from the window 56 the apparatus 50 iscommanded to switch to ranging mode by an input to the electronicapparatus 29 via keyboard 90 which is used to identify coordinates ofthe image appearing on the screen 33 designating a portion of the object28 at which range is to be measured. In switching to ranging mode thelinear drive mechanism 91 is actuated to change the zoom lens 52 intoits ranging configuration as shown in FIG. 8 which provides reduceddepth of focus in the projected beam 38. The television image currentlyon screen 33 is stored in a frame store 96 within the electronicapparatus 29 (as represented in FIG. 9) and continues to be displayed.The horizontal and vertical scanning provided by the mirrors 16 and 18is then discontinued and a much smaller amplitude horizontal dither inthe projected beam 38 is introduced by scanning of mirror 16. The extentof dither is such as to provide scanning corresponding to a width ofabout five pixels in the television image.

At the same time the electronic apparatus 29 commands the prism rotatingmechanism 87 and tube rotating mechanism to direct the projected laserbeam 38 on to the required element of the object 28 at which rangemeasurement is to be carried out. The range scanning unit 114 isactuated so as to oscillate the mirror system 62 and thereby cyclicallyvary the path length between mirror 61 and lens 68 within the scanninghead 39. This has the effect of cyclically shifting the image planepositions along the optical axis of the apparatus 50 with consequentcyclical scanning in focus distance D of the focal plane 92 at which theprojected beam 38 is focused.

The signal from the photomultiplier 27 is analysed throughout thismotion in order to detect the value of focus distance D at which signalmodulation corresponding to the speckle noise is at a maximum. Thisvalue of D is stored and is then displayed on the screen 33 on commandgiven by the operator through the keyboard 90.

The structure of the electronic apparatus 29 is illustratedschematically in FIG. 9. In its normal imaging mode the electronicapparatus 29 passes the signal from the photomultiplier 27 through anamplifier 94 to an analogue-to-digital converter 95 from which thedigitised video signal is input to a frame store 96. Each pixel of theimage is represented in the frame store by a number digitised to 8-bits.A clock signal for the analog-to-digital converter 95 is received from acontrol unit 97. The clock signal is generated by an oscillator 98 andthe clock frequency is modulated by means of a phase locked loop 99connected to the horizontal line scanning unit 72 such that the samplingof the video signal by the analogue-to-digital converter is synchronisedwith the horizontal line scan. The phase locked loop 99 providescompensation for the non-linearity of the horizontal line scan resultingfrom the approximately sinusoidal variation in rate of scan produced bymirror 16.

The control unit 97 generates analogue signals to drive the scanningunits 72,75 and 70 by means of digitally controlled drive units 100, 101and 102 respectively. A further drive unit 103 commands the rangescanning unit 114 responsible for scanning the mirror system 62 duringranging mode operation.

The control unit 97 is interconnected with a microprocessor 104 underthe command of a keyboard 90. The control unit 97 is linked to the framestore 96 via an address generator 105.

The scanning mirror 16 is oscillated at 8 KHZ and image datacorresponding to an image of 512×512 pixels is generated and held in theframe store. The television image is generated by reading the image datafrom the frame store at a scan rate which is independent of the mirrorscanning rate and is optimised to suit the television equipment used.

Switching of operation from imaging mode to ranging mode is carried outunder control of the microprocessor 104 on command received from thekeyboard 90. In ranging mode the amplified signal from thephotomultiplier 27 is passed to a rectifier 106 and an integrator 107before being routed to the analogue-to-digital converter 95. Eachdigitised signal S corresponds to the AC component of video signalintegrated over the length of scan across the object provided by thedither motion of mirror 16 and corresponding to about five pixels of theimage.

In switching to ranging mode the control unit 97 reduces the scanning ofmirror 16 to a dither by suitable command to scanning unit 72.Oscillation of mirror 18 is arrested and scanning unit 75 set to directthe mirror 18 at the required vertical coordinate at which ranging is tobe measured. The horizontal coordinate of the scanning beam is set bymeans of scanning unit 70 and mirror 69 which determines the meanposition about which horizontal scanning dither is provided by mirror16.

The control unit 97 commands the range scanning unit 114 to dynamicallyvary the focus distance D at which the projected beam 38 is focused bymovement of the mirror system 62. Since the variation of D isproportional to the square of the angular position of mirror system 62 alinear scan of focus distance is achieved by processing the commandsignal to the drive unit 103 using a look-up table 108. The position ofthe mirror system 62 is sensed using a sensor 109, the output of thesensor being digitised by an analogue-to-digital converter 110 andinterpreted as a signal representing range R by means of a look-up table111.

During ranging mode operation the screen 33 displays the last recordedimage frame entered into the frame store and this is schematicallyrepresented as an image 28 on the screen 33 in FIG. 12. Underneath theimage a horizontally scanned trace 112 representing the signal S as afunction of scanned focus distance D is displayed against a range scale113 so that the operator can interpret the peak of the trace as ameasurement of range R.

The microprocessor 104 includes software enabling the image appearing onthe screen 33 to be processed using conventional image processingtechniques as may be required.

The linear drive mechanism 91 for operating the zoom lens 52 is alsounder the control of control unit 97 and as described above the zoomlens is actuated on switching to ranging mode to provide decreased depthof field.

A number of alternative arrangements and refinements to the apparatus 50are envisaged. For example the zoom lens 52 may be actuated duringnormal viewing mode to provide increased magnification if required.

The image displayed on screen 33 may alternatively be processed underthe control of microprocessor 104 using conventional software techniquesin a number of ways. Alphameric information may be displayed containinga description of the image viewed, the image may be enhanced or portionsmagnified and the range information can be represented in any convenientform. A scale may be displayed from which the dimensions of the objectmay be measured.

The photomultiplier may be replaced by any other suitable photodetector.Analysis of the detected light may use parameters other than the noiseamplitude in the detected signal and may for example use informationderived from the spacial frequency of the speckle pattern. Software mayalso be utilised to statistically analyse the range signal using curvefitting or digital filtering techniques in order to optimise thesensitivity of the apparatus 50.

Laser light containing more than one frequency may be used duringimaging in order to obtain colour information so as to enable thedisplayed image to be in colour. Three separate lasers may for examplebe used to provide red, green and blue laser light frequencies withsuitable combination optics being incorporated in the apparatus prior toscanning of the laser beam by the laser scanning head 39. Alternativelya single laser providing a number of separate discrete wavelengths maybe utilised. The optical components can be optimised to minimiseaberrations at the wavelengths used.

The diameter of the endoscope tube may be larger or smaller than the 10mm tube of the described embodiments. In general a larger diameter isused when tubes of greater length are required and the typical range ofdiameter is between 8 and 16 mm.

The zoom lens 52 may be manually controlled as may be the prism rotatingmechanism 87 and/or the tube rotating mechanism either of which may alsobe used to adjust the direction of use during imaging.

The scanning head 39 may include scanning mirrors arranged to deflectthe beam through average angles other than 90° where this is convenientfor folding the optical axis into a more compact form.

The measurement of range R may also be utilized when the apparatus isoperating in imaging mode to adjust the focus of the projected beam soas to be focussed onto the object under inspection to provide improvedresolution.

The side viewing arrangement of apparatus 50 may be modified to provideother directions of view.

FIG. 10 shows a modification to apparatus 50 and reference numeralscorresponding to those of FIGS. 3 to 9 are used where appropriate forcorresponding elements. Modified apparatus 120 of FIG. 11 has anendoscope tube 11 with a distal end 36 terminating in an end face 121which is orthogonal to the tube 11.

A circular window 56 is provided in the end face 121 and the projectedlaser beam 38 exits from window 56 to eliminate an object 28. Lightreflected from the object 28 is received at a light collecting surface54 at a circular window 122 in the end face 121 and located next to thewindow 56.

As shown in schematic plan view in FIG. 11 the distal end 36 contains alight deflecting element 81 which is steerable by means of a rotatingmechanism 87 so that in ranging mode the projected beam 38 can besteered on to a selected portion of object 28 at which range is to bemeasured.

The deflecting element 81 is positioned between the zoom lens 52 andwindow 56 and in normal imaging mode introduces zero deflection in theprojected beam 38.

The windows 54 and 56 may be other than circular in shape. Preferablyhowever both should be of the same shape and of the same order ofcross-section.

The apparatus 50 may alternatively be modified to have a distal end 36adapted for retro viewing in which the projected beam 38 is deflected bymore than 90° from the forward viewing direction or alternatively may beadapted for oblique viewing in which the projected beam 38 is deflectedby an angle of less than 90° from the forward viewing direction.

In each case the windows 54 and 56 may be positioned side-by-side orwith window 56 located centrally and surrounded by an annular lightcollecting input surface 26 as shown for example in FIG. 1.

Apparatus 50 may alternatively include a conventional zoom lens as shownby way of example in FIGS. 12 and 13.

FIG. 12 shows a conventional zoom lens 130 incorporated in the distalend of an endoscope tube of apparatus in accordance with the presentinvention and located intermediate an optical relay 12 and a lightdeflecting prism or other light deflecting element 81.

The zoom lens 130 comprises a first stationary lens group 131 locatedimmediately adjacent the prism 81 and a second stationary lens group 132spaced intermediate the first lens group and the optical relay 12. Bothlens groups 131 and 132 are arranged to be of negative (diverging)power. An axially movable lens group 133 of positive power is locatedintermediate the first and second lens groups 131 and 132 and is movablebetween a first position as shown in FIG. 12 in which it is immediatelyadjacent the first lens group 131 and a second position as shown in FIG.13 in which it is immediately adjacent the second lens group 132.

With movable lens group 133 in its first position as shown in FIG. 12the zoom lens provides a large field angle of 50° over which theprojected laser beam 38 is scanned both horizontally and vertically.This configuration is used for imaging and provides considerable depthof field by virtue of the effective aperture of the lens being smallerthan the aperture defined by the end window 56 so that when theprojected beam is focused it has a small convergence angle.

FIG. 13 shows the conventional zoom lens 130 in its rangingconfiguration in which the field angle is reduced to 10° and the depthof focus considerably reduced by virtue of the effective aperture beingincreased to fill the end window 56 with consequent increase in theconvergence angle of the projected beam when focused.

As shown in FIG. 12 the prism 81 limits the field angle because the exitpupil of a conventional zoom lens is spaced from the prism to an extentsuch that at the maximum deflection of the projected beam 38 some of thelight is cut-off at the edges of the prism. This can be compensated tosome extent by including a larger prism 81 but only limited space isavailable within the endoscope tube 11 which must also accommodate thefibre optic bundle 53 and the prism rotating mechanism 87. It is forthis reason that the conventional zoom lens 130 is less satisfactorythan the zoom lens described above with reference to FIGS. 7 and 8.

The zoom lens of FIGS. 7 and 8 may be improved by modification to themovable lens 80 so as to have front and rear optical faces 82 and 83 ofdissimilar curvature as shown in broken lines in FIGS. 7 and 8. Such anasymmetric movable lens would no longer result in the laser beam 15being focused at one or other of the optical faces 82, 83 and this wouldreduce the extent of degradation to the optical performance of the lensdue to light scattered by any dust deposited on these optical surfaces.

The zoom lens may further be refined as shown in FIGS. 14 and 15 byreplacing the single element movable lens 80 of FIGS. 7 and 8 by amulti-component movable lens group 140 which is configured to reducecurvature of field produced in the zoom lens. The movable lens 140 mayalso be further modified to be asymmetric. In such an asymmetric lensgroup (not shown) the arrangement of elements is preferably such thatelements of negative power are followed by elements of positive power(i.e. the positive power elements are facing the fixed lens group 77).This results in the principal planes of the movable lens group 140 beingdisplaced towards the fixed lens group 77.

The zoom lens 52 can be made more effective if the travel of the movinglens group 140 between its imaging and ranging configurations ismaximised. In practice the limit of travel is set by mechanicalinterference between the movable lens group 140 and the fixed lens group77 whereas the optical relay 12 is spaced sufficiently from the zoomlens not to have limiting effect on this travel. The effect of asymmetrydescribed above allows the effective position of the movable lens group140 to be closer to the fixed lens group 77 and can thereby effectivelyincrease the available travel.

The components of the zoom lens 52 may also be corrected for chromaticaberration in known manner if it is required to use more than one laserscanning frequency or if the endoscope tube is dual purpose in that itmay also be used as part of a conventional endoscope by the attachmentof an eyepiece to the proximal end of the endoscope tube 11.

Curvature of field introduced by using a single element movable lens 80may alternatively be corrected elsewhere in the optical system of theapparatus 50. For example compensating field curvature may be introducedin the laser scanning head. Normally in the apparatus 50 of FIG. 3 thelaser beam 15 is presented as a collimated beam on to the scanningmirrors 16 and 18. If however the beam 15 is focused a short distancebefore each of the mirrors 16 and 18 such that light is diverging from apoint focus when it hits each mirror 16 and 18 then curvature of fieldresults. The field curvature introduced by the scanning head 39 and inthe zoom lens 52 can then be arranged to substantially self cancel.

A further alternative modification to the apparatus 50 of FIG. 3 isshown in FIG. 16 in which corresponding reference numerals to those ofFIG. 3 are used where appropriate for corresponding elements. Instead ofa zoom lens 52 a beam-splitter 150 is provided such that the laser beam15 is split into a first component 151 deflected through 90° to normallyexit from the endoscope tube 11 through window 56 and a second component152 directed through a fixed lens group 153 into a right angle prism 81from which it is directed through a further window 154.

shutter 155 (which is represented only schematically in FIG. 16) isprovided to allow either one of the first and second beam components 151and 152 to be selected for transmission. In FIG. 16 the shutter 155 isconfigured to allow the first beam component 151 to emerge from thewindow 56 as projected laser beam 38 and in this configuration themodified apparatus 156 of FIG. 16 is in its normal imagingconfiguration. To switch to ranging mode the shutter 155 is actuated toshut-off the first beam component 151 and transmit the second beamcomponent 152. The fixed lens group 153 comprises a diverging lensfollowed by a converging lens having the effect of broadening the beamcomponent 152 which after being deflected by the prism 81 is transmittedthrough the further window 154 as shown in broken lines in FIG. 16. Thisresults in the projected beam having an increased convergence angle andhence a reduced depth of field as required in the ranging mode. Theincreased convergence angle is accompanied by a decrease in the fieldangle so that the prism 81 is provided with a prism rotating mechanism87 so that in ranging mode the projected beam 38 can be targeted on anyrequired portion of the object 28.

The zoom lens 52 as shown in FIGS. 7 and 8 or as modified in FIGS. 14and 15 (and additionally including asymmetric movable lens and/orachromatisation as may be required) may be incorporated in aconventional endoscope 160 as illustrated schematically in FIG. 17 wherecorresponding reference numerals to those of the preceding Figures areused where appropriate for corresponding elements.

In FIG. 17 the endoscope 160 includes an endoscope tube 11 having adistal end 36 including components corresponding to those of theapparatus of FIG. 4. The endoscope tube 11 has a proximal end 35connected to a housing 161 having an eyepiece 162. The endoscope 160 isalso connected to a source of white light 163 by means of a light guide164. Light from the white light source 163 is passed into the housing161 and along a fibre optic bundle 53 to emerge from window 54 so as toilluminate the object 28. Reflected light from the object 28 is receivedat the window 56 and reflected by means of prism 81 into the zoom lens52. An image of the object is transferred to the eyepiece 162 by meansof the optical relay 12 where it is directly viewed by an observer. Thezoom lens 52 is controlled by means of a control wire 57 extending intothe housing 161 and connected to an actuator 165.

The zoom lens 52 is actuated by movement of the actuator 165 to provideswitching of the zoom lens between its two alternative configurations atwhich different angular magnifications are provided.

Various aspects of the present invention may also be incorporated in alaser scanning camera. A conventional laser scanning camera comprises alaser light source, a scanning unit for scanning the laser beam inraster fashion so as to scan an object field, a light detector fordetecting light reflected from the object field and electronic apparatusfor processing the output signal of the light detector to produce atelevision viewable image of the object field.

An improved laser scanning camera 200 is shown schematically in FIG. 18and comprises a camera body 201 housing a laser 202 and a scanning unit203. A scanned laser beam produced by the scanning unit 203 passesthrough a zoom lens 204 and finally through an end window 205 as aprojected beam 206 which scans an object field 207 in raster fashion.The drawing represents rays corresponding to maximum upper and lowerdeflection of the beam 206 and shows the zig-zag raster patterntraversed by the beam across the object field 207.

A light receiving window 208 is arranged to collect reflected light fromthe object field 207 and a photomultiplier 209 detects light transmittedthrough the window. Electronic apparatus 210 processes the output signalof photomultiplier 209 and drives a television monitor 211 whichdisplays an image of the object field 207. A keyboard 212 is connectedto the electronic apparatus 210 for the input of control commands.

The scanning unit 203 is similar to the laser scanning head 39 describedabove with reference to FIG. 5 but is modified as shown in FIG. 19 wherecorresponding reference numerals to those of FIG. 5 are used whereappropriate for corresponding elements. FIG. 19 shows the axial lightpath of the laser beam after leaving the scanning mirror 18 passingthrough a field lens pair 213 into zoom lens 204 which is representedschematically as a cylindrical component. On emerging from the zoom lens204 the light beam exits from the end window 205 towards the objectfield. The photomultiplier 209 is represented schematically as acylinder placed immediately behind the light receiving window 208.

The zoom lens 204 is constructed in the manner described above withreference to FIGS. 7, 8, 12, 13, 14 or 15 or as modified in accordancewith the various modifications discussed above.

The electronic apparatus 210 corresponds to the electronic apparatus 29described above with reference to FIG. 9 or as modified by any of themodifications described above.

In use of the laser scanning camera 200 to form an image of the objectfield 207 the camera is directed such that end window 205 faces theobject field and the laser beam 206 is projected on to the object fieldin raster fashion with the scanning unit 203 and the zoom lens 204 bothin their imaging configuration. In the imaging configuration the zoomlens provides a wide field angle of 50° and a long depth of focus. Thescanning unit 203 in its imaging configuration provides normal rasterscanning as described above.

Image information is acquired in the frame store 96 of the electronicapparatus 210 and output to the television monitor 211.

On receipt of a ranging command from the keyboard 212 the camera isswitched to its range finding mode in which the zoom lens 204 isre-configured into its ranging mode so as to provide a narrow fieldangle of 10° and a short depth focus. At the same time the scanning unit203 in its ranging mode of operation is targeted on to a selectedportion of the object field by positioning of mirror 69 and mirror 18whilst providing dither in the horizontal scan direction by means ofscanning mirror 16. At the same time the total range of the projectedlaser beam is scanned cyclically using mirror system 62.

The reflected light from the object field is detected by photomultiplier209 and processed by electronic apparatus 210 to determine the value offocus distance D at which signal modulation corresponding to laserspeckle noise is at a maximum. This value of focus distance D is storedand displayed as a measurement of range R using the monitor 211.

The method of range finding in accordance with the present invention hasapplication in all uses of endoscopy and remote inspection particularlywhere an unfamiliar scene is viewed as a television image. It does notrequire the use of a high intensity spot illumination and can bearranged to make use of the existing imaging apparatus of a laserscanning camera or an endoscopic imaging apparatus as hereinbeforedisclosed.

The imaging apparatus of the present invention also has application inall uses of remote inspection where the advantages of using a laserscanning camera can be coupled with the convenience of endoscopy. Theendoscope tube of the apparatus need not contain any active electroniccomponents and can therefore be immune to radiation hazards. The use oflaser scanning avoids the need for illumination with high intensitylight. The apparatus may be used to provide images using non visibleradiation if required.

We claim:
 1. A method of range measurement comprising the steps ofpassing a laser beam through an optical system so as to be projectedinto an object field containing an object to be ranged, detecting lightreflected from the object, varying the focus distance between an outputof the optical system and the position at which the beam is focused byoperation of a focus distance varying means, measuring a parameter ofthe detected light which is characteristic of laser speckle, determininga setting of the focus distance varying means at which the value of thespeckle parameter is consistent with the beam being focused onto theobject, and determining from calibration of the focus distance varyingmeans the corresponding value of focus distance as a measurement ofrange, wherein the beam is directed onto a selected area of the objectand scanned across the selected area, the reflected light being detectedby means of a photodetector producing an electrical output signal, thespeckle parameter being measured as the output of circuit meansresponsive to noise in the output signal and the presence of maximumnoise in the output signal being taken as being consistent with the beambeing focused onto the object.
 2. A method as claimed in claim 1 whereinthe focus distance varying means comprises a mirror system which ismovable to vary the path length travelled by a converging or divergingportion of the beam.
 3. A method as claimed in claim 2 wherein the focusdistance varying means comprises at least one axially movable lens inthe path of the beam.
 4. A method as claimed in claim 1 in which rangeis measured by means of apparatus comprising the optical system, theapparatus being selectively operable to generate a television image ofthe object field by scanning the beam in raster fashion, the apparatusbeing switchable between ranging mode and imaging mode as required, andwherein the apparatus includes means operable to vary the depth of focusof the beam such that in the ranging mode the depth of focus is lessthan in the imaging mode.
 5. A method as claimed in claim 4 wherein thedepth of focus is varied by actuation of a zoom lens of the opticalsystem which provides a field angle at the output of the optical systemwhich is reduced in the ranging mode relative to the field angleprovided in the imaging mode.
 6. A method as claimed in claim 4 whereinthe depth of focus is varied by passing the beam through differentlenses in the ranging mode and imaging mode respectively.
 7. A method asclaimed in claim 4 wherein the apparatus further comprises a scanninghead including at least one pivoting mirror, the scanning head beingoperable in the imaging mode to deflect the laser beam into an input ofthe optical system such that the angle between the beam and optical axisis scanned in raster fashion, a corresponding raster scan being producedin the beam projected from the output of the optical system to scan theobject, and the scanning head being operable in the ranging mode toprovide scanning of the beam only over a selected area of the object. 8.A method as claimed in claim 4 wherein the beam is deflected towards theselected area by light deflecting means operable to vary the directionof the optical axis at the output of the optical system when theselected area would otherwise be located outside of the reduced fieldangle of the optical system in the ranging mode.
 9. A method of rangemeasurement comprising the steps of passing a laser beam through anoptical system so as to be projected into an object field containing anobject to be ranged, detecting light reflected from the object, varyingthe focus distance between an output of the optical system and theposition at which the beam is focused by operation of a focus distancevarying means, measuring a parameter of the detected light which ischaracteristic of laser speckle, determining a setting of the focusdistance varying means at which the value of the speckle parameter isconsistent with the beam being focused onto the object, and determiningfrom calibration of the focus distance varying means the correspondingvalue of focus distance as a measurement of range, wherein the opticalsystem further comprises an optical relay, the method including the stepof passing the laser beam through the optical relay, which optical relaydefines an optical axis extending longitudinally through an elongatetube, the output of the optical system being located at a distal end ofthe tube which is insertable into confined spaces for range measurementof inaccessible objects.
 10. Imaging apparatus comprising an opticalsystem defining an optical axis, a scanning head connected to an inputof the optical system, a laser light source connected to an input of thescanning head, the scanning head being operable to project a laser beaminto the input of the optical system such that the angle between thebeam and the axis is scanned in raster fashion, the optical systemhaving an output comprising a light transmitting window from which thelaser beam is projected towards an object field, a light receivingwindow located adjacent the transmitting window for collecting lightreflected from the object field, detection means producing an electricaloutput signal responsive to light collected by the light receivingwindow, and electronic apparatus operable to produce a television signalfrom the detector means output signal whereby a television image of anobject in the object field may be obtained, wherein the optical systemcomprises an elongate tube, the transmitting window and the receivingwindow being mounted in a distal end of the tube, the optical systemincluding an optical relay through which the optical axis extendslongitudinally within the tube, and wherein the tube is of smallcross-section relative to the scanning head so as to be insertable intoconfined spaces for imaging inaccessible objects.
 11. Imaging apparatusas claimed in claim 10 further comprising ranging means operable tomeasure the range of an object in the object field under inspection. 12.Apparatus as claimed in claim 11 wherein the ranging means comprisesmeans varying the focus distance between the light transmitting windowand the position at which the beam is focused and circuit meansresponsive to noise characteristic of laser speckle in the output signalof the detection means, the electronic apparatus being operable todetermine a setting of the focus distance varying means at which maximumnoise is present in the output signal and to determine from calibrationof the focus distance varying means the corresponding value of focusdistance as a measure of range.
 13. Apparatus as claimed in claim 12wherein the electronic apparatus includes a frame store containing imageinformation of each pixel of the television image, the frame storeinformation being refreshed at the scanning rate of the scanning headand the television signal being derived from the information in theframe store at a scanning rate which is independent of the scanninghead.
 14. Apparatus as claimed in claim 13 wherein the electronicapparatus provides for an image generated by a previous scan in imagemode operation to be stored in the frame store during operation of theapparatus in ranging mode such that a television image may continue tobe displayed.
 15. Apparatus as claimed in claim 10 wherein the detectionmeans comprises a fibre optic link extending between the light receivingwindow and a photodetector at the proximal end of the tube. 16.Apparatus as claimed in claim 11 wherein the scanning head is removablyconnected to the endoscope tube, the tube being connectable to aneyepiece for use in direct viewing through the optical system. 17.Apparatus as claimed in claim 11 wherein the optical system includes azoom lens located in the distal end portion of the tube and means forremotely actuating the zoom lens.